1. Field of the Invention
The present invention relates to a film bulk acoustic resonator (FBAR) filter, and more especially to an FBAR filter operating at microwave frequencies which can be simply fabricated using thin film technologies.
2. Description of the Related Art
It is important to prepare filters at high frequency in the MHz or GHz region because those frequency regions are often used for wireless communications these days. For such applications it is important that the filter is as small size as possible. Therefore, filter configurations are demanded which minimise the size of the filter on the substrate surface.
FBARs are attractive devices since they show resonant peaks at high frequency particularly in the MHz and GHz regions. Moreover, FBARs can be achieved in a small device (size ˜100 microns). Thus, they are considered to be useful for installation in small, light, thin electric appliance products, such as mobile phones.
FBARs are fabricated by depositing thin film piezoelectric layers, typically zinc oxide (ZnO), aluminium nitride (AlN) or lead zirconium titanate (PZT) onto a membrane or acoustic quarter wavelength stack formed on a semiconductor substrate. This combination forms an acoustic structure which is resonant at a specific frequency.
Prior art FBAR devices are coupled to associated circuitry by means of contact pads and wire bonds. This method provides a poorly defined ground connection because of the parasitic inductance of the wire leads. This approach also makes simulations and analysis of results difficult.
An alternative approach of incorporating the FBAR within a coplanar waveguide (CPW) structure involves the signal connection of the ground-signal-ground connection making contact to the top and bottom of the piezoelectric layer. In the known arrangement the electrodes are separate from the transmission line structure. This increases the area and limits the size of device that can be produced.